This invention relates to a catadioptric system. More specifically this invention relates to a two mirror telescopic objective having a field lens corrector group.
In conventional mirror telescope systems, it is known to provide a field group corrector lens having a positive net power. The purpose of the corrector group is to speed up the f number of the two mirror system, which has a relatively slow value, to the higher f number value required by the overall system. However, because the Petzval curvature aberration of such a positive field group has the same sign as the aberration of the two mirror system, the net aberration of the overall system is quite undesireable.
In order to provide good aberration correction, the prior art systems were required to compound the positive field group into several different elements, each of which had strongly curved negative surfaces. This was necessary to correct for aberrations while maintaining a positive net power to speed up the f number of the system.
Thus, the conventional two mirror system disadvantageously increases the Petzval curvature type aberration for the overall two mirror system as it increases the f number of the transmitted rays. This requires the use of many lens elements in the field group having strongly curved negative surfaces, thus increasing the cost and complexity of the system.
It is incidentally noted that my own prior U.S. Pat. No. 3,489,487 shows an example of a single mirror system useful in photography and electronic processing having a spherical aberration corrector component and a corrector component for compensation for under correction of coma and astigmatism from the single mirror.
It is an object of the present invention to provide a two mirror telescopic system having good Petzval curvature aberration correction characteristics while simultaneously increasing the f number of the system.
It is a further object of the present invention to provide such a two mirror system having a long back focus, low obscuration characteristics, covering a wide field of view, and comprising a minimum amount of components.